1. Field of the Invention
The present invention relates to a stencil printing machine in which a stencil sheet subjected to a stencil making process is attached to and wound around a rotary cylindrical drum.
The present application is based on Japanese Patent Application No. 2000-180153, which is incorporated herein by reference.
2. Description of the Related Art
Generally, a stencil printing machine has a rotary cylindrical drum in which an ink-permeable circumferential wall with a porous structure is arranged in a hollow cylindrical shape, and which is rotatable about its own axis. A stencil sheet subjected to a stencil making process is attached to and wound around the circumferential wall of the rotary cylindrical drum. An ink supplying device for supplying ink to an outer peripheral surface of the circumferential wall to allow the ink to permeate therethrough is disposed in the rotary cylindrical drum. Disposed on the outer side of the rotary cylindrical drum is a pressing device for pressing a printing sheet against the outer peripheral surface (the attached stencil sheet) of the circumferential wall. The printing sheet is fed into a nip between the rotary cylindrical drum and the pressing device in synchronism with the rotation of the rotary cylindrical drum, and the printing sheet is pressed against the stencil sheet attached to the rotary cylindrical drum so allow the ink to pass through perforated portions of the stencil sheet and to be transferred to the printing sheet, thereby effecting printing.
In this type of stencil printing machine, the stencil sheet is supported by being wound in roll form. The stencil sheet drawn out from the roll of stencil sheet has the shape of a continuous belt. This drawn-out stencil sheet is subjected to a stencil making process (i.e., thermographic perforation) by a thermal head. At the time of stencil making, the stencil sheet is conveyed while being guided into a nip between upper and lower guide plates and while coming into contact with the thermal head by the rotation of a platen roller opposing the thermal head. Further, a leading end of the stencil sheet subjected to a stencil making process is conveyed to a retaining member provided on the outer peripheral surface of the rotary cylindrical drum by pairs of load rollers. The retaining member clamps and fixes the leading end of the stencil sheet which has been conveyed. After the leading end of the stencil sheet is fixed by the retaining member, the stencil sheet is attached to and wound around the outer peripheral surface of the rotary cylindrical drum as the rotary cylindrical drum rotates. In addition, when the stencil sheet is attached to the rotary cylindrical drum, tension is imparted to the stencil sheet, whose leading end is fixed by the retaining member, by the nip of the pairs of load rollers. Then, the stencil sheet is cut into a stencil portion by a cutter unit while being attached to the rotary cylindrical drum.
However, with the above-described stencil printing machine, when the stencil sheet is attached to the rotary cylindrical drum, tension is imparted to the stencil sheet by the nip of the pairs of load rollers, but the stencil sheet assumes a state in which the tension is not imparted to it after a trailing end of the stencil sheet cut by the cutter unit has passed through the pairs of load rollers. Further, the trailing end of the stencil sheet tends to be oriented toward the outer peripheral surface of the rotary cylindrical drum owing to the tension imparted up until then. As a result, when the stencil sheet is attached to the rotary cylindrical drum, the stencil sheet is attached in such a manner that the trailing end of the stencil sheet is lifted from the outer peripheral surface of the rotary cylindrical drum. Hence, there has been a problem in that wrinkles are formed in this portion as it is pressed by the pressing device during printing, appearing as an image on the printing sheet.
It should be noted that the load rollers for imparting tension to the stencil sheet, including their driving mechanism, cannot be installed close to the rotary cylindrical drum in view of the need to avoid interference with the retaining member when the rotary cylindrical drum is rotated and interference with the retaining member which is operated when the retaining member retains the leading end of the stencil sheet.
Accordingly, it is an object of the present invention to provide a stencil printing machine which makes it possible to install the mechanism for imparting tension to the stencil sheet close to the rotary cylindrical drum and to prevent the occurrence of wrinkles in the stencil sheet attached to the rotary cylindrical drum, thereby overcoming the above-described drawbacks of the related art.
To achieve the above object, according to a first aspect of the present invention, there is provided a stencil printing machine which comprises: a rotary cylindrical drum rotatable about its own axis, the rotary cylindrical drum having a retaining member which retains, at an outer peripheral surface of the rotary cylindrical drum, a leading end of a stencil sheet which has been subjected to a stencil making process, wherein the stencil sheet is wound around the outer peripheral surface of the rotary cylindrical drum in accordance with a rotation thereof; a pair of upper and lower guide plates which guide conveyance of the stencil sheet to the rotary cylindrical drum; a swinging member having a swinging end portion which is vertically swingable while the swinging member is downwardly urged so that the swinging end portion is oriented toward the rotary cylindrical drum and located adjacent to an end portion of the upper guide plate oriented toward the rotary cylindrical drum; and a supporting member attached to an underside of the swinging member, the supporting member supporting the stencil sheet being conveyed, while nipping the stencil sheet in cooperation with the lower guide plate.
With the stencil printing machine in accordance with the first aspect of the present invention, the stencil sheet whose leading end is retained by the retaining member provided on the rotary cylindrical drum is attached to and wound around the rotary cylindrical drum as the rotary cylindrical drum rotates. At this juncture, the stencil sheet is conveyed while being guided by the upper and lower guide plates, and is nipped by the supporting member and the lower guide plate, and predetermined tension is applied to the stencil sheet by an urging force of the swinging member. Since the position for nipping the stencil sheet by the supporting member and the lower guide plate is at distal end portions of the upper and lower guide plates facing the rotary cylindrical drum, the tension is applied to the stencil sheet until immediately before the delivery of the stencil sheet from the upper and lower guide plates to the rotary cylindrical drum. As a result, it is possible to impart tension to the stencil sheet being attached to and wound around the rotary cylindrical drum in a range from the leading end to the trailing end of the stencil sheet, thereby making it possible to prevent the occurrence of the wrinkles when the stencil sheet is attached to the rotary cylindrical drum.
According to a second aspect of the present invention, in the stencil printing machine of the first aspect, it is preferable that when the retaining member is in an open state in order to retain the leading end of the stencil sheet, the retaining member abuts against the swinging end portion of the swinging member so that the swinging member swings, and in accordance with a swinging motion of the swinging member, the supporting member is moved away from the lower guide plate.
With the stencil printing machine in accordance with the second aspect of the present invention, when the retaining member is in an open state in order to retain the leading end of the stencil sheet, the swinging member is arranged to swing as the swinging end portion abuts against the retaining member, and the supporting member is moved away from the lower guide plate in conjunction with the swinging motion. Consequently, when the leading end of the stencil sheet is retained by the retaining portion, the stencil sheet can be guided without hampering the conveyance of the stencil sheet between the upper and lower guide plates.
According to a third aspect of the present invention, in the stencil printing machine of the first or second aspect, it is preferable that the supporting member includes a foamed elastic member, and the swinging member includes an elastic film.
With the stencil printing machine in accordance with the third aspect of the present invention, a simple arrangement is provided in which the supporting member is formed of, for example, a foamed elastic material, and the swinging member is formed of, for example, an elastic film. Therefore, the arrangement for obtaining the above-described effect can be applied to the upper and lower guide plates of an existing stencil printing machine without modifying a large number of component parts.
According to a fourth aspect of the present invention, in the stencil printing machine of the first or second aspect, the upper guide plate may have an insertion hole through which the supporting member is insertable, and in this case, the supporting member may be brought into contact with the stencil sheet through the insertion hole when the supporting member supports the stencil sheet being conveyed.